A primary driving force in the hard disk drive (HDD) industry is the need for greater areal density. Historically, areal density has increased between 30% and 100% annually, through improvements in both track (radial) density and recording (circumferential) density. This increase in areal density has resulted in hard disk drives with significantly higher capacity and has also contributed to dramatic decreases in cost per megabyte.
The most obvious impact of greater areal density on the hard disk drive servo system is the increased performance required to follow data tracks that are shrinking in width. However, another important effect is that as the data bits shrink, the read/write heads must fly closer to the disk surface. Current head-to-disk spacing is approximately 6 nm, compared to approximately 60 nm ten years ago. Because the disk surface is not perfectly flat and the slider fly-height varies, contact can occur between the disk and the slider that contains the read/write heads, causing off-track motion of the read/write heads. For example, if there is a reduction in atmospheric pressure, as might be experienced by a mobile or lap-top computer being used on an airplane, the fly-height can be reduced by a significant percentage, resulting in head-to-disk contact and off-track motion (track misregistration) that can significantly impact read/write operations. In the past, head-disk contact was rare and has been dealt with as isolated events that often caused temporary interruption of the read/write operations. In the future, contact may occur more regularly or perhaps almost constantly, as in the case of contact recording.
Prior Art FIG. 1A illustrates a slider 110 on a suspension 107 that contains a read/write head 112 flying above an ideal flat disk surface 115 with a fly-height 130. It should be understood that this figure, as well as FIGS. 1B, 1C and 1D, is purely for illustrative purposes and is not intended to be to scale.
Prior Art FIG. 1B illustrates a disk surface 115 that has a small topographical irregularity 140. Although topographical irregularity 140 is shown as an isolated “bump” or “bubble” on disk surface 115, it could be any number of configurations of disparities, such as, for example, ridges that might result from disk clamping.
Prior Art FIG. 1C illustrates slider 110 on a suspension 107 containing a read/write head 112 contacting topographical irregularity 140 as fly-height 130 becomes zero.
Prior Art FIG. 1D illustrates the off-track writing that might occur when slider 110 containing the read/write head contacts disk surface irregularity 140. If read/write head 112 contained by slider 110 had been writing on track 145, for example, upon contacting topographical irregularity 140 it might jump from track 145 to track 155, as illustrated by path 150.
One of the most common types of repetitive errors, also known as repetitive runout or RRO, occurs when the disk slips in relation to the center of rotation of the spindle motor. This results in errors that typically have their largest amplitude at the first, second, or third harmonics of the spindle rotation speed, which is relatively low in frequency compared to the bandwidth of the feedback control system. The conventional method of compensating for errors encountered repeatedly on a disk is to add a peak filter to the feedback controller at the desired spindle harmonics. This increases the open loop gain at the desired spindle harmonics, which improves the disturbance rejection at these frequencies. Although this technique degrades the disturbance rejection at other frequencies, there is generally an overall improvement in the positioning accuracy. Due to stability requirements, use of peak filters is generally limited to frequencies below the open loop bandwidth of the control system. Feedforward techniques have also been proposed for compensating RRO, but these techniques have also typically been applied to low frequency disturbances.
This is illustrated in Prior Art FIG. 2. Prior Art FIG. 2 is a block diagram of a conventional feedback loop for compensating for position errors on a disk. A disturbance 225 occurs that affects the position of the hard disk drive actuator 220 at junction 227. This disturbance results in measured position 230 of the HDD actuator. Reference position signal 205 joins measured position signal 230 containing a position error created by disturbance 225 at junction 207. These signals become a position error signal (PES) 210 and enter feedback controller 215 which adjusts the signal to hard disk drive actuator 220 to compensate for disturbance 225. The feedback controller 215 design may include peak filters as described previously. This typical feedback loop does nothing to prevent the disturbance 225 from having an initial impact, but tries to compensate for the disturbance after its effects appear in the PES. The feedback loop does work to minimize the effects of the disturbance. However, it is generally not able to provide sufficient positioning accuracy in the presence of head-to-disk contact-induced disturbances, especially in the case where the disturbance frequency is above the bandwidth of the feedback system. For large amounts of track misregistration that might occur from head-to-disk contact, this conventional feedback loop is inadequate.